The Study On Improving The Heat Transfer Performance Of Ground Heat Exchangers

With the development of economy more and more attention has been paid to the environmental problems, and sustainable policy has been becoming a household word across all over the world. Owing to their features of energy saving and environment- friendliness, ground-source heat pump (GSHP) systems for HVAC(Heating Ventilating and Air Conditioning) have aroused greater and greater interest in China in recent years. The GSHP system, which has been widely used in developed countries in Europe and North America, is an instrument that utilizes the vast geothermal energy stored in the earth for the heating and air conditioning in commercial and residential buildings. The GSHP system is quite different from conventional HVAC system in its ground heat exchanger (GHE), which is buried underground to absorb or dissipate heat. Moreover, it has great significance to improve the heat transfer performance of GHE from/to its ambient soil so as to design the size of GHE reasonably, ensure the high efficiency of the system and keep the initial cost and maintenance cost as low as possible. Thus, the crucial task of the research and application of GSHP is to develop ways of improving the heat transfer performance of the GHE.Based on analytical and experimental studies detailed analysis and discussion have been undertaken in this thesis on how to improve the heat transfer performance of the GHE.Firstly, factors influencing heat transfer performance of the GHE has been analyzed, and the corresponding methods are pointed out, including adopt inclined borehole disposition to increase the actual spaces among boreholes, improve the thermal conductivity of the backfill, and choose antifreeze solutions with lower freezing point and so on. These measures may help to lessen the total length of the GHE and, then, to reduce the first cost of the system,Secondly, in allusion to inclined borehole disposition GHE, analytical solutions of both the steady-state and unsteady-state temperature distribution have been derived in a semi-infinite medium with an inclined line-source of finite length on the basis of virtual heat source method and linear superposition principle. The impacts of geometrical and physical parameters on the heat transfer process have been discussed accordingly. Two representative steady-state temperatures of the inclined borehole wall are defined as the middle section average temperature and the whole-length integral average temperature. Differences between them are evaluated, and concise expressions for both of them are presented for practical engineering applications. Steady-state temperature distributions around multiple inclined boreholes are analyzed. Compared with corresponding vertical boreholes, the thermal interaction among multiple inclined boreholes is much less.Finally, laboratory studies have been undertaken to optimize various performances of silica sand-cement backfill grouts through formula design and tests. The advanced Transient Plane Source (TPS) technique is used to study the influence of fillers and admixtures to the thermal conductivities of cementitious grout materials. Other performances were also examined, such as pumpability, expansibility and so on. High-performance grout formulas have been developed on the basis of domestic materials with thermal conductivity above 2.1 W/(m.K). A case study shows that application of these new types of cement-sand grouts can reduce the size and, then, the first cost of ground heat exchangers significantly compared with conventional bentonite grouts, especially in sites of hard-rock strata with high thermal conductivities.